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Lately the targeting on my site has improved dramatically. I don't know why exactly -- I've re-done a lot of the navigational structure but my sense is that the adsense also went through some internal changes lately as well.
Anyway, as the result I now have higher CTR but much lower earnings per click. Which sort of makes sense if you think about it -- there is probably less competition for more specific terms than for global broad coverage terms.
All in all it resulted in a about 15% drop in actual ernings per impression (CPM). Go figure why I spent a month re-doing navigation :)
I tell myself that I've improved the experience of my users (and I really believe I did) -- and that'll cover my expenses in the long run.